Milk Factors Controlling the Plasma Calcitonin Level in the Newborn Rat*

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Abstract
The elevated plasma calcitonin (CT) levels found during suckling in the newborn rat, in contrast with the low or undetectable values found during fasting, prompted this investigation of the effect of milk constituents on plasma CT. Plasma concentrations of calcium, inorganic phosphorus, glucose, insulin, and CT were determined in 2-h-old newborns which had previously received (120, 90, 60, or 30 min before) by stomach gavage triglycerides, lactose, casein enzymatic hydrolysate, or calcium in physiological amounts. Triglycerides (20 mg in 0.2 ml) induced a rapid and sustained increase in plasma CT levels without change in plasma calcium. Blood glucose levels increased, but plasma insulin changed very little. A transient increase in plasma CT was observed 30 min after lactose administration (20 mg in 0.2 ml). This stimulation (∼2-fold rise) was, however, smaller than that observed after triglycerides (∼6-fold rise). Blood glucose and insulin both were markedly increased throughout the time course of the lactose experiment. Plasma CT and calcium did not change after the fasted newborn rats were fed casein enzymatic hydrolysate (20 mg in 0.2 ml), however, phosphate increased 30 min later. Plasma insulin was greatly increased 30, 60, 90, and 120 min after the gavage with casein enzymatic hydrolysate, but the increase in plasma glucose was equivocal until 60 min after this gavage. A dose of calcium given orally (200 mg/kg BW), close to the amount in stomach contents (milk curds), appeared to elevate plasma CT 5, 10, and 20 min later in response to increased plasma calcium. The specificity of these changes in response to milk constituents was supported by the use of control fasted newborn rats fed 0.2 ml 0.9% NaCl. The plasma concentrations of calcium, inorganic phosphorus, glucose, CT, and insulin in these controls did not change during the course of the experiment. These data demonstrate the part played by the fat, lactose, and calcium present in milk as stimuli (through some unknown gastrointestinal hormone) for the secretion of CT during suckling in the rat.